Sanchez said the cause was still being investigated. A passenger, Don Yarbrough, told KUSA TV of Denver that investigators believed the victims reacted to a cleaning fluid containing ammonia that had been used on the plane after it arrived in Denver from Cancun, Mexico.

A United spokeswoman in Chicago, Robin Urbanski, said victims felt weak and nauseous but she had no other details.

There were 143 passengers and crew on the plane when it landed in Las Vegas.